MINERALOGY. IO3 



lire are not unconimon, and it is often possible to prove them to be micro- 

 cline mixed with ortlioclase. It is probable that they are always par- 

 tially microcline. The Roxbury and Troy granites furnish examples in 

 which quite large grains exist, which can be demonstrated to be micro- 

 cline. Des Cloizeaux shows that albite is also commonly interlaminated in 

 smaller or greater amounts, recognizable by the small angle at which, in 

 albite, the extinction of light takes place between crossed Nicols. The 

 green color of these crystals has been supposed to be due to the presence 

 of a trace of copper, of protoxide of iron, and of organic matter. Analysis 

 detects no copper, but a little heat destroys the color, which supports 

 the idea that it is due to a minute amount of some organic substance. 



64. Tourmaline [(K,^ Na,^ IV) (Mg, Fe)^ (AI,^ B^)^ Si^ 0=°]. 



Tourmaline is a very common mineral, and also offers some peculiar- 

 ities in our state. It is hexagonal, and is found in three-sided prisms, 

 which by the multiplication of prismatic faces, become sometimes nearly 

 cylindrical. When well crystallized it is usually terminated by the three 

 planes of the rhombohedron — 3 R, or by a combination of this with other 

 rhombohedrons. It is generally black ; but light brown crystals are 

 also found, which are more highly prized on account of their rarity. The 

 great granitic veins are the most noteworthy localities, though tourma- 

 lines are scattered through all the old schists, and are common in some 

 granites. 



At Springfield very large crystals are found, which are quite unique 

 from the extraordinary development of the basal plane. Fig. 3 on PI. 3 

 is drawn from one of them. The crystals, which are short and thick, and 

 possess the planes represented in the figure, sometimes reach the great 

 size of five inches in diameter, still maintaining their perfection and 

 habit. They occur in the granitic veins. 



The best brown tourmalines are found at Orford, where they have crys- 

 tallized in a bed of steatite, and often are of a large size. Crystals six 

 inches long and two in diameter have been found, but more often it 

 occurs in radiating masses of imperfect form. Brown tourmaline, in a 

 massive condition, is found at Warren. 



Blue and green tourmaline, called indicolite, is found at Hinsdale, and 

 also abundantly in a granite near Winchester. 



